Conventional control systems for multiphase rotary electric machines are designed to drive a plurality of switching elements connected to a multiphase rotary electric machine as a controlled object to thereby control rotation of the multiphase rotary electric machine.
One example of such conventional control systems is disclosed in Japanese Patent Application Publication No. 2005-102350.
A control system for a three-phase rotary electric machine disclosed in the Japanese Patent Application Publication is designed to drive a pair of high and low side switching elements for each phase winding of the machine to output a sinusoidal PWM (Pulse Width Modulated) voltage to be applied to each of the three-phase windings. This allows torque to be created in the three-phase windings with little ripples.
The control system of the patent application Publication focuses on the fact:
When the low side switching elements are all on, the three-phase windings are short-circuited to a negative terminal of a battery. In this state, an induced voltage in one phase winding of the three-phase rotary electric machine at a timing when a phase current to flow through the one phase winding becomes zero depends on the amount of change in the phase current. A timing of a wave signal changed to become zero will be referred to as “zero crossing” hereinafter.
Specifically, at the zero crossing of the phase current, the zero crossing of the amount of change in the phase current is matched with that of an induced voltage in the one phase winding. The zero crossing of the induced voltage has a predetermined positional relationship with respect to the rotational position of the rotor.
Based on the fact, the control system is designed to:
measure a phase current to flow through each of the three-phase windings when the low side switching elements are all on;
match the zero crossing of the measured phase current to flow through each of the three-phase windings with that of the amount of change in the corresponding phase current; and
determine the zero crossing of an induced voltage in one of the three-phase windings based on the zero crossing of the amount of change in the phase current to flow through the one of the three-phase windings to thereby determine rotational positions of the rotor based on the determined zero crossing of the induced voltage.
As described above, the zero crossing of an induced voltage in one of the three-phase windings is associated with rotational positions of the rotor of the three-phase rotary electric machine. For this reason, it is possible for the control system to control rotation of the rotor based on the determined rotational positions of the rotor without using sensors for measuring the rotational positions of the rotor. Such rotation control of a rotor of a multiphase rotary electric machine without using sensors for measuring rotational positions of the rotor will be referred to as “sensorless control” hereinafter.
Another example of such conventional control systems is disclosed in U.S. Pat. No. 6,456,030 corresponding to Japanese Examined Patent Publication No. 3598909.
As set forth above, the control system disclosed in the patent application Publication needs to measure a phase current to flow through each of the three-phase windings only when the low side switching elements are all on. This reduces periods during which the phase current to flow each of the three-phase windings can be measured as a modulation factor of the sinusoidal PWM voltage increases, causing the measurement of the phase current to become difficult. For this reason, the more the modulation factor of the sinusoidal PWM voltage increases, the more the difficulty of performing the sensorless control may increase.